1. Field of the Invention
The invention relates generally to optical communications systems. In particular, the invention relates to optical regenerators in optical communication systems.
2. Background Art
Communication networks are increasingly being implemented with optical fiber as the transmission medium. Optical fiber, particularly silica fiber, offers many advantages of high bandwidth, low noise, and relatively low loss. The total bandwidth can be further increased by wavelength division multiplexing (WDM) in which multiple optical carriers of wavelengths differing by about a nanometer or less are modulated with separate data signals and are then multiplexed onto a single optical fiber.
Much effort has been expended in reducing the loss due to absorption in optical fiber. As a result, optical signals can be transmitted over many kilometers without the need for amplification or regeneration of the signals. Nonetheless, there is some loss, particularly if the fiber is spliced. Further loss occurs in all-optical switching nodes, including passive couplers, which can be inserted into the optical path to eliminate the need to convert the signal between optical and electrical forms for switching. Accordingly, many optical communication networks, particularly those spanning significant distances, include optical amplifiers which can amplify the optical signal without the need for converting it to electrical form. Erbium-doped fiber amplifiers are commonly used for such optical amplification. They are particularly useful because they have a substantial gain bandwidth sufficient to simultaneously amplify a large number of WDM wavelengths without the need to demultiplex them before amplification.
Optical amplification should be contrasted with conventional regeneration in which the modulation of a transmission signal is detected and the extracted data is used to modulate a new transmission carrier, whether it be electrical or optical. In optical transmission, the conventional regeneration requires an optical to electrical (O/E) conversion and an electrical to optical (E/O) conversion.
Optical amplification enjoys additional advantages over conventional regeneration because it is independent of the data format. For example, Yoo has proposed an optical router in U.S. patent application Ser. Nos. 09/654,384 and 10/081,396, filed respectively on Sep. 2, 2000 and Feb. 22, 2002, now issued as U.S. Pat. Nos. 6,519,062 and 6,768,827, both incorporated herein by reference in their entireties. Such optical routers allow packets to be routed across a complex optical network according to addressing information contained in their headers without the need to decode the data portion. Optical amplifiers can amplify such optical packets without decoding any portion of the packet or even knowing the packet format. However, optical amplifiers have a non-flat gain spectrum and produce noise. Therefore, amplification alone does not prevent degradation of the data waveform and its timing.
Loss, however, is not the only limiting factor for fiber transmission. Depending upon the chosen transmission wavelength and the particulars of the fiber, there may be some chromatic, waveguide, or other temporal dispersion, which broadens the wave form of the optical data signal. Further, although fiber noise is low, some noise becomes impressed on the optical data signal so that square data pulses are degraded. Sources of noise include non-linearities and cross-talk. Accordingly, reshaping of the optical pulse is desired. Yet further, the timing of the pulse train may become degraded, for example, by jitter in the transmitter or in other elements (perhaps caused by temperature variation), temporal dispersion in the fiber, indefinite pulse edges due to shape degradation, polarization mode dispersion (PMD), polarization dependent loss (PDL). Simply reshaping the optical pulses does not completely address the timing problem. Conventional electrical regenerators reshape binary signals to nearly their original form and provide a new, reclocked signal in a process often referred to as retiming or reclocking. Conventional optical amplifiers do not reshape or retime optical data signals. It is nonetheless desirable to accomplish the regeneration, reshaping, and reclocking of optical signals, often referred to as 3R regeneration, without converting the optical signals to electrical form.
Amplification or regeneration is additionally desirable in complex optical networks receiving input signals from disparate sources over different transmission paths and lengths but switching them through a common switching or routing fabric. Further, the source power of the distant transmitter may vary over time. It is desirable that the switched signals be of nearly equal amplitude or power to allow common optimization of the switch fabric. Electronic regeneration performs this power equalization. Isolated optical amplifiers require close control to effect dynamic power equalization.
As a result, even optical networks would benefit from regeneration. However, the standard electrical regeneration requiring optical-to-electrical (O/E) conversion and E/O conversion for each of the WDM channels as well as decoding the packet format does not scale well with a large number of WDM wavelengths.
Much recent work has addressed 3R regeneration of optical signals. However, many of the approaches involve complex optical systems, often in conjunction with analog electronic components, which are not suited for commercialization. It is greatly desired to provide optical regeneration in an integrated system amenable to mass production.